Multi-stage chlorine dioxide delignification of wood pulp

ABSTRACT

A process of producing papermaking pulp by partially digesting wood followed by mechanical refining and then delignifying with chlorine dioxide in three stages with caustic extraction between the stages, the amount of chlorine dioxide in each of the chlorine dioxide stages being progressively smaller and the total amount of chlorine dioxide used being not more than about 9% based on the dry wieght of the original wood.

United States Patent Wilder 1 1 *Nov. 11, 1975 154] MULTI-STAGE CHLORINE DIOXIDE 2.382.140 5/1968 Henderson cl a1 1621011 X [)ELlGNI ()F WOOD PULP 2.44am); 5.1969 Asplund e1 163/1 3.591.451 7. 1971 \Nilder 162, 67 1 Memo" Wilder- Mldlothiun- V11 muss? 3.11973 W'ildcr.......... 1mm x [73] Assignecz Ethyl Corporation Richmond Va 3.829.357 8/1974 1 hompson ct .11.. 16212-1 l Notice: The portion of the term of this FOREIGN PATENTS OR APPLICATIONS patent subsequent to Jul 6 1988 553.129 2/1958 Cunnda 16289 has been dlsclaimed. OTHER PUBLICATIONS [22 F'led: Levitin and Schwartz. Delignilication of Spruce Saw- [21] Appl. N() j 309,601 dust with Chlorine Dioxide in Pulp and Paper Maga- Related US. Application Data Continuation-impart of Ser. No. 193.220. Oct. 27. 1971. abandoned. which is a continuation of Ser. No. 889.662. Dec. 31. 1969. abandoned. and a continuation-impart of Ser. No. 121.574. March 5. 1971. abandoned. said Ser. No. 889.662. is a continuation-in-part of Ser. No. 797.209. Feb. 6. 1969. Pat. No. 1591.451. said Ser. No. 121.574. is a division of Ser. No. 797.209.

zine of Canada. Vol. 55 (Jan. 1954) pp. 92-98.

Primary E.\u/m'|m'S. Lcon Bashore Ass/stun! E.\um1'ucr--Arthur L. Corbin [57] ABSTRACT A process of producing papermaking pulp by partially digesting wood followed by mechanical refining and 7 Claims, 12 Drawing Figures US. Patent Nov.1l, 1975 Sheet10f9 3,919,041

FIBER CHPS PRETREATMENT f CHLORlNE DIOXIDE H TREATMENT WATER WASHING 2 CAUSTIC EXTRACTION l 3 TREATMENT WATER WASHING 6 PULP FIG. I.

US. Patent Nov. 11, 1975 FIBER CHIPS PRETREATMENT .1

CHLORINE DIOXIDE TREATMENT WATER WASHING CAUSTIC EXTRACTION WATER WASH IN G WATER WASHING CAUSTIC EXTRACTI Sheet 2 of 9 WATER WASHING CHLORINE DIOXIDE v TREATMENT 40 FRESH WATER -q WATER WASH IN G PULP FIG. 2.

U.S. Patent Nov. 11, 1975 Sheet 3 of9 3,919,041

FIBER CHIPS CHEMiCAL PRETREATMENT REFINING ,-7|

WATER WASHING 'r2 FIG.3.

U.S. Patent Nov. 11, 1975 shw 4 of9 3,919,041

r s w a w n 2522 BEEEE 6 wad;

NOOH APPLIED lN EACH STAGE,7.0F PULP FIG. 4.

w uztaim u 0 NOOH APPLIED IN EACH STAGE, 7. OF PULP FIG.5.

US. Patent Nov.11, 1975 Sheet50f9 3,919,041

Q mhuuwum 3.0 4.0 5.0 NoOH APPLIED,'A OF PULP F l G. 6.

U N M m M TA IAE Jam wa Rm P *T a R 1/ K NEUTRAL SULFITE PRETREATMENT 0 2 mmmmaw a swmu4 CZ. dd; 22: 855

I00 PRETREATMENT YIELD'I. (X)

40 7O 8O 9O FIG.7.

SCHOPPER FOLD m4 awe 5 O0 000 DAYS AT FIG. IO.

MULTI-STAGE CHLORINE DIOXIDE DELIGNIFICATION OF WOOD PULP The present application is a continuation-in-part of application Ser. No. 193,220, filed Oct. 27, 1971 subsequently abandoned, which in turn is a continuation of application Ser. No. 889,662, filed Dec. 31, 1969 subsequently abandoned, and a continuation-in-part of application Ser. No. 121,574, filed Mar. 5, 1971 also subsequently abandoned, said application Ser. No. 889,662 being a continuation-in-part of application Ser. No. 797,209, filed Feb. 6, 1969 US. Pat. No. 3,591,451 granted July 6, 1971, and said application Ser. No. 121,574 being a division of application Ser. No. 797,209.

BACKGROUND OF THE INVENTION Vegetable materials such as wood, reed, bamboo, cane and the like which are or can be used for the preparation of fibrous materials are composed of several basic parts. In general, fibrous vegetable matter is made up of about 15 to 30 percent lignins and extractives, such as resins and the like, with the remainder, about 70 to 80 percent, being carbohydrates and including hemicellulose, alpha cellulose and other celluloses. Based on the weight of the fibrous vegetable matter, about 10 to 30 percent is hemicellulose about 45 to 55 percent is alpha cellulose and about percent other celluloses.

As is well known, one of the first steps in conventionally converting fibrous vegetable materials to fibers for use in the preparation of paper or paper-like materials is a pulping process. The primary goal of the process is to remove most of the lignins from the fibrous vegetable material and separate the remaining carbohydrate fibers into individual fibers. In all known pulping processes, such as kraft, sulfite and others, when efforts are made to remove substantially all of the lignin from the vegetable fiber mass, a major part of the hemicellulose is lost, and the remaining cellulose and hemicellulose fibers are chemically and/or mechanically damaged. This results in a significant loss of yield and a major reduction in strength of paper or paper-like products due to fiber damage. For example, in a kraft pulping process, the normal yield known in the art is about 45 percent by weight. But, if only the lignins and extractives were removed from the fibrous vegetable material, the yield obtained would be 70-80 percent.

SUMMARY OF THE INVENTION This invention is directed toward a new and novel pulping process and the pulp and paper products resulting therefrom. The pulping process removes substantially only lignins and extractives from fibrous vegetable materials and leaves the cellulose and hemicellulose part of the material substantially undamaged, thereby resulting in pulp and paper products having new and unusual properties and exceptionally high strengths. Since the pulping process is reasonably selective and substantially only the lignins and extractives are removed, yields are exceptionally high and in the 55 to 85 percent range. The pulping process includes a basic sequential unit of a chlorine dioxide treatment, caustic extraction and a chlorine dioxide treatment, and this basic unit is preceded by a chemical or mechanical pretreatment of prepared vegetable fiber chips. Each step of the basic sequential unit is followed by a water washing, which may be with process liquids obtained elsewhere in the process, as by countercurrent washing which has attendant conservation advantages.

A more preferred embodiment of the process is one including a chemical and/or mechanical pretreatment of prepared vegetable fiber chips along with refining at elevated temperature and pressure followed by the sequential processing of the pretreated-refined chips in a chlorine dioxide treatment, a caustic extraction. a chlorine dioxide treatment, a caustic extraction and a final chlorine dioxide treatment. A water washing or its equivalent follows each chlorine dioxide treatment and each caustic extraction.

An even more preferred embodiment of the process is one in which the water wash for the final chlorine dioxide treatment is used as the water wash for the preceding caustic extraction and so on countercurrently to the flow of fiber material through the process to the first water wash following the first chlorine dioxide treatment; from this point the wash water may then be sent to waste or treated for recovery of chemicals contained therein.

Another preferred embodiment of the process involves the chemical pretreatment of prepared vegetable fiber chips followed sequentially by a chlorine dioxide treatment, a caustic extraction, a chlorine dioxide treatment, a caustic extraction and a final chlorine dioxide treatment with countercurrent water washing and extraction after each treatment; the most preferred chemical pretreatment is a neutral sulfite pretreatment at a specified concentration of chemicals and cooking cycle.

The pulp produced by the process of the present invention has a higher degree of polymerization, a higher hemicellulose content, a higher carboxyl content, and a lower carbonyl content than conventionally bleached kraft pulp from the same wood mixture. In addition, it requires less energy to refine than a conventionally bleached kraft pulp from the same wood mixture. Also, laboratory handsheets prepared from this pulp have superior tensile strength and tear strength when compared to sheets of conventionally bleached kraft pulp from the same wood mixture.

The above pulp properties relate directly to paper made from the pulp. Such paper has higher tensile, tear, burst, fold, pick, and delamination strengths.

BRIEF DESCRIPTION OF THE DRAWING In the drawing, FIGS. 1 to 3 are block diagrams and FIGS. 4 to 13 are graphs.

FIG. 1 describes the basic process of this invention;

FIG. 2 shows a more preferred embodiment of the process of this invention;

FIG. 3 discloses a highly preferred chemical pretreatment step for the process of this invention;

FIG. 4 compares yield of pretreated material with caustic utilized in each stage;

FIG. 5 compares G. E. brightness with caustic utilized in each stage;

FIG. 6 compares percent rejects with caustic utilized;

FIG. 7 shows final yield compared with pretreatment yield;

FIG. 8 compares chlorine dioxide consumption with pretreatment yield;

FIG. 9 shows a comparison of freeness with beating time;

FIG. 10 compares Schopper Fold with effect of cation treatment;

3 FIG. ll provides a comparison of MIT Fold with Schopper Fold;

FIG. [2 relates Schopper Fold to days of aging; FIG. l3 relates Schopper Fold to years of aging.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, fiber chips of any fibrous vegetable matter are fed first to a pretreatment step which may be either mechanical, chemical or a combination thereof. Following pretreatment, the pretreated chips are then fed to a first chlorine dioxide treatment II where they are contacted with chlorine dioxide in either aqueous solution or as a gas. Following this, the chlorine dioxide treated material is washed with water 12 to return the mixture to substantially neutral pH; following washing, the washed chlorine dioxide treated material is subjected to caustic extraction 13 for a period of about one-half to one hour. The extracted material is washed again with water to return to substantially neutral pH, as indicated at 14, and to remove the water soluble materials produced in the extraction step. Then this second washed, extracted fibrous material is subjected to a second chlorine dioxide treatment 15, either aqueous or gaseous, and the second chlorine dioxide treated material is water washed, 16, to produce a pulp in high yield with good brightness. Yields from this basic process range from about 55 to about 85 percent at a G. E. color brightness (TAPPI Standard T 217 m-48) of approximately 80 to 90.

Referring now to FIG. 2, a more preferred embodiment of the invention is shown. Prepared vegetable fiber chips are fed to a pretreatment step where they are subjected to a chemical, mechanical or a combined chemical-mechanical pretreatment to make the lignin and extractives more readily available for removal. Following the pretreatment step, the pretreated material is subjected sequentially to a chlorine dioxide treatment 21, water washing 22, caustic extraction 23, water washing 24, chlorine dioxide treatment 25, water washing 26, caustic extraction 27, water washing 28, chlorine dioxide treatment 29, and water washing 30.

In a more preferred embodiment, fresh water is fed only to the last water washing 30 by line 40 and then circulated countercurrently to the flow of the material through the process as indicated by line 41 to the next to last water washing step 28, and from there as indicated by line 42 to the next preceding water washing step 26, then by line 43 to water washing step 24, and from that wash to the first water washing 22 as indicated by line 44, and then to waste or chemical recovery as indicated by line 45. It is clear, of course, that fresh or make-up water may be added to any one of the water washing steps as indicated by lines 46, 47, 48, 49 and that water may be sent to waste or chemical recovery from any or all of the water washing steps as indicated by lines 50, 51, 52 and 53.

Chlorine dioxide, either in aqueous solution or in gaseous form, may be fed to each of the chlorine dioxide steps as indicated by lines 60, 61 and 62 and an aqueous solution of caustic may be fed to each of the caustic extraction steps as indicated by lines 63 and 64. The novel pulp of this invention is recovered from the process. as indicated at 70, in a high yield of about 55 to about 85 percent and at a G. E. brightness of approximately 80 to 90.

A more preferred embodiment of the invention is a five stage process that comprises the sequential steps of 4 chlorine dioxide treatment, caustic extraction, chlorine dioxide treatment, caustic extraction, and chlorine dioxide treatment, with a water wash between each step, and having first chlorine dioxide treatment preceded by chemical or a chemical-mechanical pretreatment.

Referring now to FIG. 3, a block diagram of a preferred chemical-mechanical pretreatment is shown. Prepared vegetable fiber chips are fed to a chemical treatment step where a pre-pulping treatment is given. This prepulping treatment may be in the nature of a weak pulping to a high yield by a kraft, nitric acid, neutral sulfite process, or other. In the prepulping treatment, the fiber chips are prepulped to a yield at least greater than 64 percent by weight based upon the dry weight of the chips of the vegetable matter. Following the chemical treatment, the prepulped material is refined in step 71, washed in step 72, then dewatered as indicated at 73 to prepare a combined chemicallymechanically pretreated material ready for processing in the first chlorine dioxide treatment as indicated at 21 in FIG. 2.

The novel process of this invention is suitable for preparing a novel pulp and paper product from any fibrous vegetable matter containing lignin. As is necessary with all pulping processes, the vegetable matter should have extraneous materials removed before being subjected to the process. For example, in the case of wood, it must be debarked in a prior operation. In the following description, wood will be referred to as the fibrous vegetable material; however, it should be understood that the process of the invention is applicable to all fibrous vegetable materials.

Debarked wood, either hard or soft, may be converted into chips by a Carthage multiknife chipper or other equivalent apparatus. The chips should be approximately l5 to 75 millimeters in length, ID to 40 millimeters in width and have a thickness of 0.5 to 20 millimeters. When chemical or chemical-mechanical pretreatment is used it is preferred that chips have an approximate length and width as described and that the thickness be from about 2 to about 5 millimeters. Following chipping, prepared chips are then subjected to the pretreatment step.

The pretreatment step can be either mechanical, chemical or a combination of chemical and mechanical. In mechanical pre-treatment, the vegetable fiber chips are subjected to a shredding, refining, or flaking operation, such as is well known in the art, by a Fallman knife ring flaker, which by slicing reduces conventionally sized chips to thin flakes while maintaining chip length and width, or a standard disc refiner, or the equivalent. As is also known, the chips may be subjected to water or steam treatment prior to flaking or refining, either under vacuum or pressure, and following either flaking or refining the resulting fibers or fiber bundles should be as small as possible without significant damage to the fibers. The optimum size depends upon the flaking or refining equipment employed. When chemical pretreatment is used, the vegetable fiber chips are subjected to a chemical treatment followed by a refining operation and then a water washing. The chemical pretreatment results in a yield of at least about 64 percent or greater and may be a mild prepulping by a neutral sulfite, nitric acid, kraft or other known pulping process (e.g., bisulfite, acid sulfite, cold soda, soda, sodium xylene sulfonate, polysulfide). A more preferred chemical pretreatment is a mild neutral sulfite prepulping under particular conditions of chemical concentrations; the heating and cooking cycles are defined infra. After either the chemical or mechanical pretreatment and refining step, a dewatering step may be necessary prior to subjecting the pretreated fibers to the novel pulping process of this invention.

The refining step may be performed by standard disc refiner or other equivalent apparatus and conducted to yield minimum particle size without significant fiber damage. While it is desirable to refine the chemically pretreated chips as much as possible, so that subsequent washing can be more efficiently achieved and reactions with chlorine dioxide can proceed more uniformly and result in a more uniform, shive-free pulp, it is however clear that too much refining at this stage will not only break up the wood structure into fibers as desired but will also damage and break up the individual fibers. This will tend to reduce the quality of the finai bleached pulp, which is undesired. The primary objective therefore is to break up the wood structure but not the fiber structure so that the particle size after refining is as fine as possible, consistent with minimizing fiber damage. In accordance with a highly preferred embodiment of this invention, this objective is successfully achieved by refining under elevated temperature, preferably in a pressurized disc refiner. A temperature of from about 212F to about 400F at a corresponding saturated steam pressure of from about pounds per square inch absolute to about 245 pounds per square inch absolute is suitable; a temperature of from about 212F to about 290F at a corresponding saturated steam pressure of from about 15 pounds per square inch absolute to about 60 pounds per square inch absolute is preferred. Temperatures up to about 212F can be achieved in a conventional disc refiner. Higher. preferred temperatures require a pressurized refiner. Temperatures above preferred in the pressurized refiner tend to reduce pulp quality. A power input of from about 0.2 to about 10 horsepower-days per ton of dry pretreated chips is suitable; a power input of from about 0.5 to about 5 horsepower-days per ton of dry pretreated chips is preferred. There is a pronounced saving in the power requirement for pressure refining as compared with conventional refining, in addition to improvement in pulp quality. Generally, there is no need to use higher power inputs with the pressurized refiner since the resultant refined material is finely divided and well suited for washing and for reacting efficiently and rapidly in the chlorine dioxide sequence of this invention.

It has been found that when using a chemical pretreatment in preference to only a mechanical pretreatment, the amount of fines produced in the refining is reduced, the optimum diameter of the fiber bundles produced is reduced, the energy input to the refining operation is reduced, the quantity of chlorine dioxide necessary for pulping to a desired brightness is reduced, the quality of the final pulp from the novel process is improved, and the yield of the pulp from the final process is increased.

Following pretreatment, either mechanical or chemical, as the case may be, and refining, the pretreatedrefined material enters the first chlorine dioxide treatment step. In this step the shredded mass of fiber bundles resulting from the pretreatment-refining has a consistency of from about 5 percent to about 50 percent by weight, based on the total weight of shredded mass and water. Chlorine dioxide, if used as an aqueous solution,

may be fed as an approximately l percent by weight aqueous solution, and depending upon the desired concentration of chlorine dioxide, which is defined infra, additional water may be added to prepare the mixture to the desired consistency. lf gaseous chlorine dioxide is used, an inert diluent such as air may be necessary to prevent explosion hazards.

Any conventional treating tower such as is well known in the art may be used for the first chlorine dioxide treatment stage and heat may be added if and as necessary. Also, additional heat may be supplied to reduce the time of contact between the shredded mass and the chlorine dioxide, which time is from about 10 minutes to 2 hours depending upon the consistency, the temperature, and the yield of product resulting from the pretreatment step. Normally no external heating is required, and the reaction is carried out at a temperature between about 5C and about C. Preferably this temperature is between about 20C and about 40C. In general, the shredded mass of fibers is permitted to remain in contact with the chlorine dioxide until the chlorine dioxide charged is substantially consumed. The pH of this system at the beginning may vary from about 4.0 to about 8.0, and upon consumption of the chlorine dioxide the pH of the treated solution will be approximately 0.5 to 3.0. Following the chlorine dioxide treatment, the resulting mass is then water washed in a conventional vacuum drum washer or the equivalent.

Following the first water washing, and when the material has a substantially neutral pH, the washed material is subjected to a first alkali extraction in a conventional treating tower such as is well known in the art. in the alkali extraction, any water soluble caustic material may be used such as sodium hydroxide, ammonium hydroxide, sodium carbonate, ammonia gas or other or mixtures of these or others; however, an aqueous solution of sodium hydroxide is preferred. In the extraction, the alkali application should be approximately 4 percent based on the oven dry weight of the fibrous material, and sufficient water may be added or removed to prepare an aqueous fiber mass having a consistency of from about 5 percent to about 50 percent by weight based on the total weight of shredded mass present and water. The alkali extraction should continue for at least about one-half hour at a temperature of from about 50C to about C with a preferred temperature of about 65C. Following alkali extraction, the alkali extracted material is subjected to another water washing under substantially the same conditions as the first water wash to remove extracted materials and residual chemicals.

The second chlorine dioxide treatment may be carried out in a conventional treating tower such as described for the first chlorine dioxide treatment, whereby the desired consistency of material within the tower is substantially the same for the second chlorine dioxide treatment as for the first. Either gaseous chlorine dioxide or an aqueous, approximately one percent by weight, solution may be fed to this second treatment stage. In this stage the pH is initially from about 4.0 to about 8.0 and ends at about 2.0; the chlorine dioxide treatment is permitted to continue until substantially all the chlorine dioxide charged to the treating stage is consumed. The temperatures for the second chlorine dioxide treatment are preferably adjusted to from about 40C to about 60C to keep contact times to a minimum of from about 30 minutes to about 4 hours to consume the chlorine dioxide charged. Following the second chlorine dioxide treatment, the treated material is subjected to a third water washing under substantially the same conditions as the first and second water washings. After the third water wash, a second alkali extraction is conducted, followed by a water wash under substantially the same conditions as the first alkali extraction and wash. The washed material at this stage in the process may be screened, if desired, to remove any shives of fibrous material which may remain, and these shives are discarded or returned to the first chlorine dioxide treatment stage for recycle.

The treated material, whether screened or not. is then subjected to a third chlorine dioxide treatment under the same conditions of consistency and chlorine dioxide concentration as the first and second chlorine dioxide treatment stages for a period of from about 2 hours to about 6 hours, depending upon the desired brightness for the product produced. The temperature for this third chlorine dioxide treatment stage is preferably from about 40C to about 80C, and following the third chlorine dioxide treatment, the treated material is subjected to a fifth and final water wash under the same conditions as the preceding water washings.

The total concentration of chlorine dioxide used in the multistage process, whether two, three or more chlorine dioxide stages, is dependent upon the yield of product obtained from the pretreatment step and the desired brightness of the product resulting from the final treatment stage. in general, the total chlorine dioxide consumed in the multiple stages, regardless of the number of stages used, is from about 1.0 to about l5.0 percent by weight based on the total dry weight of fibrous material being fed to the pretreatment stage. it has been found and is preferred that the total concen tration of chlorine dioxide used is from about 4.0 percent to about 13.0 percent by weight, based upon the total weight of dry fibrous material being fed to the pretreatment stage.

The amount of chlorine dioxide fed to each chlorine dioxide stage is dependent upon the number of chlorine dioxide stages used and on the pretreatment yield. For any given total amount of chlorine dioxide to be used, it has been found that approximately two times the amount used in the last stage should be fed to the chlorine dioxide stage preceding the last and two times the amount used in the preceding stage fed to the next preceding stage, and so on, For example, in a three chlorine dioxide stage process, this means that approximately four-sevenths of the total chlorine dioxide will be fed to the first stage, approximately two-sevenths of the total chlorine dioxide will be fed to the second stage, and approximately one-seventh to the third stage.

As mentioned previously, the preferred pretreatment for the process of this invention is a chemical pretreatment, and of the chemical pretreatments available such as kraft, bisulfite, neutral sulfite, nitric acid, etc., a neutral sulfite pretreatment is preferred. And, among the neutral sulfite prctreatments available, a sodium based neutral sulfite pretreatment is preferred. As well known in the art, a standard neutral sulfite pulping treatment includes cooking fibrous vegetable material for a period of 10 to l5 minutes at about 350F in a concentration of approximately l0 percent sodium sulfite and approximately 3 percent sodium carbonate, chemical charges being based on the wood weight charged to the process. Although this standard neutral sulfite pretreatment has advantages, in the process of this invention it is even more preferred that a specific and novel neutral sulfite pretreatment be used. This novel chemical pre treatment includes preparing an aqueous solution of a fiibrous vegetable material, which has been chipped as described previously, with a concentration of from about 5 to about 30 percent sodium sulfite and from about 3 to about 25 percent sodium carbonate to provide a sodium sulfite to sodium carbonate ratio of about 1.2 or greater, More preferred concentrations are from about 7 to about 20 percent sodium sulfite and from about 5 to about 18 percent sodium carbonate, all percentages being based upon the dry weight of the vegetable matter. A more preferred sodium sulfite to sodium carbonate ratio is from about 1.2 to about [.5. The time'temperature relationship employed is designed to give adequate impregnation of liquor into chips prior to reaching a temperature of about 300]? This relationship is dependent upon wood species and chip size, as well as previous chip history. When a chemical pretreatment is performed in accordance with the described recipe, higher final yields and higher quality product are obtained as compared with other mechanical or chemical pretreatments.

in the following description, all evaluations of paper and pulp products were made, unless otherwise indicated, at a standard basis weight of 40 pounds per 3000 square feet.

The pulp of the present invention is chemically unique in that it has a higher degree of polymerization, a higher hemicellulose content, a higher carboxyl content, and a lower carbonyl content than pulp conventionally produced from the same wood. Due to its higher final yield as compared to conventionally bleached kraft pulp, it contains more hemicellulose. At the same time, however, the viscosity average degree of polymerization of the pulp is higher than that of conventionally bleached kraft pulp. The inescapable conclusion is that the process of the present invention degrades wood cellulose less than conventional processes in going from wood to purified pulp. Thus, the carboxyl content of the pulp produced by the present invention is at least twice as great as the carboxyl content of conventionally bleached pulps and has a carboxyl number (TAPPI Standard T 237 Eli-63) greater than about 6, preferably greater than about 9, and more preferably greater than about 12, and as high as, for example, 20, and even higher. At the same time, the carbonyl con tent is only one half to one-third as great as the carbony] content of conventionally bleached pulp from the same wood.

It is well established that the brightness stability of a pulp is related to the carbonyl content of the pulp. The higher the carbonyl content, the greater the brightness loss during aging. Since the product of the present invention possesses a very low pulp carbonyl content, it is quite stable and loses little brightness with aging. By comparison, the higher carbonyl content of pulps not produced by the present invention result in rather poor brightness stability.

The mechanical properties of the pulp of the present invention which are affected by its unique chemical properties are case of refining, fiber tensile strength, and ability to form fiberfiber bonds when made into sheets and dried. The pulp of the present invention consumes only one-third to one-fourth of the energy required to beat a conventionally bleached kraft pulp from the same wood mixture to the same freeness level. The rate of beating of the pulp of the present invention is 4.2 times as great as the rate for the corresponding bleached kraft pulp when prepared from a northern hardwood mixture and 3 times as great using the southern hardwood mixture. Since the time required to beat a pulp is directly proportional to the energy required to beat that pulp, the pulp of the present invention exhibits a substantial savings in refining energy input required to reach a given freeness level. The rate of mechanical refining (ml. Canadian St. per minute of beating carried out according to TAPPl Standard T 200ts- 66) can be greater than about 15, preferably greater than about 20, more preferably greater than about 25, and may extend up to, for example, 50 and even higher. When the pulp of the present invention is formed into paper on a paper machine, more rapid drainage, increased ability to retain fibers, increased web web strength, and increased drying rate are observed relative to conventional pulp prepared from the same wood.

Strips of handsheets from the pulp of the present invention possess superior tensile strength and tear strength when compared to conventionally bleached kraft pulp from the same wood mixture. This is unusual since pulps with higher tensile strength usually possess lower tear strength. The fact that the present pulp possesses both a superior tear strength and superior tensile strength indicates another unique physical property of the pulp of this invention; it also possesses superior individual fiber tensile strength.

The properties of these pulps relate directly to papers made from them. Machine made paper from pulp of E efficiency factor R carbohydrate retention factor the present invention produces higher tensile, tear, burst, fold, pick and delamination strengths. The grease proofness (TAPPI Standard T 454ts-66) of the paper of this invention can be greater than about 500 sec., preferably greater than about 1000 sec., and can extend up to, for example, I800 sec. and even higher; tensile strength (TAPPI Standard T 404ts-66) for paper from hardwood pulp can be greater than about 80 percent, preferably greater than about I percent, more preferably greater than about I percent and may extend up to, for example, 200 percent and even higher, and for paper from softwood pulp it can be greater than about 120 percent, preferably greater than about I40 percent, more preferably greater than about 160 percent, and may extend up to, for example, 250 percent and even higher; bursting strength (TAPPI Standard T 403ts-63) for paper from hardwood pulp can be greater than about I40 percent, preferably greater than about I60 percent, more preferably greater than about I90 percent, and may extend up to, for example, 250 percent and even higher, and for paper from softwood pulp it can be greater than about I60 percent, preferably greater than about 190 percent, more preferably greater than about 230 percent, and may extend up to, for example, 300 percent and even higher; tearing resistance (TAPPI Standard T 4l4ts-65) for paper from hardwood pulp can be greater than about I60 percent, preferably greater than about 220 percent, more preferably greater than about 300 percent, and may extend up to, for example, 400 percent and even higher, and

for paper from softwood pulp it can be greater than about 320 percent, preferably greater than about 370 percent, more preferably greater than about 420 percent, and may extend up to, for example, 600 percent and even higher; folding endurance (MlT fold, TAPPI Standard T 423su-68) for paper from hardwood pulp can be greater than about 500, preferably greater than about I000, more preferably greater than about I500, and may extend up to, for example, 3000 and even higher, and for paper from softwood pulp it can be greater than about I000, preferably greater than about 2000, more preferably greater than about 4000, and may extend up to, for example, 6000 and even higher.

The novel process of this invention may be understood better by reference to the following examples; however, it should be understood that these examples are intended to be descriptive rather than restrictive.

EXAMPLE I To demonstrate the advantage of using multiple chlorine dioxide-caustic extraction stages in delignification as opposed to a single chlorine dioxide stage, experiments were carried out using finely ground southern hardwood meal, which was then reacted with aqueous chlorine dioxide solutions at an initial pH 4 and at 70C for a reaction time of one hour, which led to complete consumption of the chlorine dioxide.

Following the reaction-extraction sequence, overall yield and lignin content (Klason) were determined in relation to each sample. Carbohydrate content was cal culated by difference. Two factors were then defined:

grams Klason lignin removed grams chlorine dioxide applied and Optimum conditions result with a high value of E, while maintaining an R value as close to unity as possible. By comparison, a bleached hardwood kraft pulp would have an R value of approximately 0.55.

The table below shows the results of these experiments for a single chlorine dioxide stage, a single stage followed by an extraction, and a three-stage sequence. It will be noted that the inclusion of the extraction stage (second case) more than doubles the efficiency while not greatly reducing the retention or selectivity factor. Since the product following any extraction stage is very dark, a final chlorine dioxide stage is required to produce a bleached product. The high E and relatively high R values are maintained through this second chlorine dioxide stage, demonstrating the superiority of the multistage approach.

Sodium Hydroxide Extraction- S% Chlorine Dioxide With the chlorine dioxide-extraction-chlorine dioxide sequences shown above, the resultant product is not of as high a brightness as it is possible to achieve. Also, when a three-stage sequence is applied to a fibrous ma- 1 l terial obtained from either a mechanical or chemicalmechanical pretreatment of wood (as opposed to the fine wood meal referred to above), the resulting pulp contains some shives or fiber bundles. Use of more 12 cases, chlorine dioxide application percentages are based on starting pretreated material.

chlorine dioxide in each of the two stages as well as use 5 g il rf 8223:: i mf g gs of a more severe extraction stage improves results. fifi Dioxide hi Dioxide However, this leads to an increase in chlorine dioxide Stage 511186 Yield s consumption and a decrease in R. Thus, the preferred i L 40 32.0 method of producing a bleached pulp with a negligible 25 i quantity of shives is conducted by expanding the se- 4 1 quence to five stages of intermittent chlorine dioxide- 5 4.0 L0 77.9 3.6 extraction (with intermediate washing). The additional ,TAPH Standard T 222 My stages give the caustic an additional opportunity to soften and disperse the fiber bundles, and also to remove further alkali-soluble lignin materials and thereby is h hgnm dam the table agam Show reduce the overall chlorine dioxide consumption. cluswely that opnmum practice cans for more than one-half the 5 percent chlorine dioxide to be applied in EXAMPLE 1] the second chlorine dioxide stage; there is no signifi- To demonstrate the preferred distribution of chlorine cam change m hgmn free W P Runs 4 and dioxide between stages, hardwood chips were pre- From two "P ?"9 It P h from treated chemically by a sodium base neutral sulfite rethe stfmdpomt chlorine dmx'de f action to approximately 85 percent yield, followed by Sumpuo" and m l 2 f f (bleached) mechanical refining in an 8 inch laboratory disc refiner the total chlorine dioxide application should be distriband thorough water washing uted between stages in the approximate proportions of Representative samples of the resultant pretreated four'sevemhs in s,tage two'sevgmhs in stage material were subjected to various chlorine dioxide exand one'sevemh In stage three traction sequences. The following conditions were held EXAMPLE "1 constant during these tests:

To demonstrate the advantage of using sodium hydroxi rather than s dium c n te r mm ium First Chlorine Dioxide Stage: 10% consistency; reaction to o arbo a o a on exhaustion; hydroxide in the extraction stages, the following com- First Caustic Stage: 4% Sodium hydroxide based on parisons were made, demonstrating that sodium hygfig i z zz rzgggf droxide is more effective in extracting lignin reaction Second Chlorine Dioxide Stage: 10% consistency; 65C reaction products than either sodium carbonate or ammonium second Caustic Stage: f f g' g hydroxide. At the same time, sodium hydroxide is at calm: least as effective as the other two extractants in pre- Third Chlorine Dioxide Stage: sfiine coitiiditions as in second servi g pulp carbohydrat content. c mm stage The first runs below compare sodium hydroxide and sodium carbonate as extractants. Mixed southern hard- Water a as employe after all 3 First, the 40 wood chips were pretreated chemically by a neutral distribution between the first two chlorine dioxide sulfite treatment and mechanically in a laboratory disc stages was studied by five combinations of chlorine direfiner to a final pretreatment yield of 84 percent. Foloxide in these stages, holding the total amount applied lowing thorough water washing, the pretreated material constant at five percent, based on starting pretreated was pulped with 5.5 percent chlorine dioxide (wood m rial. limin i g h in l hir hl r dioxide basis), extractions were carried out at 10 consistency stage and making yield and lignin determinations. The and 65C, and the effect of variable extraction condiresults are shown below: tions was determined through yield and lignin content Chlorine Dioxide Chlorine Dioxide '70 Run Applied, First Stage Applied, Second Stage Yield Lignin' 1 L0 4.0 85.4 14.2 2 2.0 3.0 86.0 10.6 3 2.5 2.5 36.4 8.7 4 3.0 2.0 86.8 8.8 5 4.0 i.0 86.3 8.9 Base 0 0 100 18.7

new sniiomi T 222 iii-s4.

From the above table, it is apparent that maximum dfhgmficatfon i at least half of the measurements on the washed pulp. The extraction con- P chlonfle f f' bemg m the i stage ditions used and experimental results were as follows: maximum yield is indicated with about twice as much chlorine dioxide in the first stage as in the second. Equiv This type of experiment was repeated using the five- Amt 5, stage sequence (chlorine dioxide-extraction-chlorine A s odiurn raclcld, liigninfi iqiyrime, dioxide-extraction-chlorine dioxide with a constant g s z gg; gg; m; application of 4 percent chlorine dioxide in the first Chemical Pulp ide hr. material material material stage and 5 percent distributed between the second and mo" 4 4 1 79.8 2" third chlorine dioxide stages as shown below. In all Na,C0, 5.6 4.2 I 33.3 5.3 78.0

-continuecl Equiv. Amt. of Ex- Carbo- Sodium trac- Yield. Lignin, hydrate. Amt, Hytion prepreit preii of droxtime, treated treated treated Chemical Pulp ide hr. material material material Na,CO, 5.6 4.2 4 80.4 4.l 76.3

TAPPl Standard T 222 m-54.

It is evident that equal extraction time results in a much lower lignin extraction efficiency with the carbonate than with the hydroxide. Also, the hydroxide removes no more carbohydrate than does the carbonate. An increase in carbonate extraction time increases the lignin extraction efficiency, but does not give results equivalent to hydroxide. Longer carbonate extractions result in increased carbohydrate extraction, which is undesirable.

Similar runs were made using ammonium hydroxide and sodium hydroxide. The starting material was a 73 percent neutral sulfite-disc refiner pretreated hardwood mixture, with a 4 percent chlorine dioxide pulping stage preceding the extractions. The extraction conditions and results are shown below.

'TAPPI Standard T 222 til-$4.

It is apparent from these results that ammonium hydroxide extraction, even at the higher level of chemical application, is less efficient in lignin extraction. Also, ammonium hydroxide offers no advantage in carbohydrate preservation.

EXAMPLE IV To demonstrate the preferred conditions of temperature and applied alkali to be used in the caustic extraction stages of the chlorine dioxide-extraction sequence, a pretreated wood such as produced in Example X was delignified using the following four-stage sequence:

5% chlorine dioxide (wood basis). consistency, one hour;

First Chlorine Dioxide Stage:

First Extraction Stage:

Second Chlorine Dioxide Stage:

Second Extraction Stage:

Water wash was employed after each of the above. The resulting pulps were analyzed for yield, G.E. brightness (TAPPI Standard T 2l7 m-48), and screen rejects (amount retained on a 0.006-inch laboratory vibratory flat screen). The results for two extraction temperatures (20 and C) and an applied alkali ranging from I to 6 percent (pulp basis) are shown in the drawing in FIG. 4 (yield), FIG. 5 (brightness), and FIG. 6 (rejects).

FIGS. 4 and 5 show that yield losses and brightness are relatively little affected by temperature. However, while yield loss is approximately proportional to alkali applied, there is little advantage in either brightness increase or rejects reduction in using more than about 4 percent sodium hydroxide at 12 percent consistency. Therefore, to conserve yield while achieving maximum brightness and minimum rejects, about 4 percent applied caustic is employed at 12 percent consistency. FIG. 6 clearly shows that a higher temperature (65C) is advantageous in reducing rejects. Since this does not adversely affect yield, preferred temperature conditions for extraction are above ambient, in the vicinity of 65C.

When using the high temperature (2| 2F to 400F) refining step referred to above, there is less difference in the results obtained at different caustic extraction temperatures, and very good products are obtained with any extraction temperature that is not so severe as to cause significant degradation of the pulp.

EXAMPLE V To show the preferred range of sodium base neutral sulfite pretreatment yield, a series of sodium base neutral sulfite pretreatments was carried out. In all cases, sufficient (and constant between runs) liquor impregnation of the southern hardwood chips was allowed before the pretreatment temperature was raised to its maximum of 335F. Following the chemical pretreatment, the hardwood materials were refined in an eightinch laboratory disc refiner and thoroughly washed. The materials were then pulped and bleached to G. E. brightness (TAPPl Standard T 217 m-48) by the preferred five-stage chlorine dioxide-caustic extraction sequence, using the total amounts of chlorine dioxide corresponding to the level of pretreatment yield involved and shown in Example IX. The pretreatment yield range covered was 6] percent to 86 percent.

The yield data, strength data of standard laboratory handsheets, and some data on the degree of polymerization (D.P.) of the final bleached pulp is summarized in the following table.

At 600/300 Degrees -continued At 600/300 Degrees Neutral Sulfite Final Bleached Canadian Standard Freeness Pretreatment yield. of '17 yield, of wood wood DP Tensile Burst Tear 7t 57 70/89 llS/l59 -/l75 86 65 2100 55/90 96/l78 -/l76 DPtpolymerizationl estimated from Cuene viscosity measurement, performed according to TAPPl Stan- It is evident from these data that there is a trend toward increasing (all) strength properties as pretreatment yield is increased. Also, it is evident that this increase generally is most pronounced over the yield range of 61 to 70 percent, and then a reasonably constant, high strength level is maintained with further yield increases.

This same trend is borne out by the degree of polymerization (DP) data. As bleached yield decreases (other things remaining constant), a pulp contains less low-DP hemicellulose and (proportionately) more high-DP cellulose. Therefore, if there were no undesirable chemical degradation occurring, a steady increase in DP would be expected as pretreatment yield decreases. Actually, just the opposite occurs, showing that the chemical pretreatment (particularly in the pretreatment yield range below 70%) is having a chemically degrading effect on the retained pulp carbohydrates.

From both the test results and the measurements of pulp DP or molecular length, it is apparent that maximum pulp strengths are favored at higher chemical pretreatment yields. Further, this effect is most pronounced at the low end of the pretreatment yield scale, and suggests that the preferred conditions would involve pretreatment yields above approximately 64 per cent.

An upper limit on pretreatment yield is fixed by considering the pretreatment yield-final bleached yieldchlorine dioxide consumption relationships. FIG. 8 shows the pretreatment yield-chlorine dioxide consumption relationship for neutral sulfite pretreatment, with the no-chemical pretreatment point corresponding to I00 percent pretreatment yield. In reality, the neutral sulfite line is followed until a pretreatment yield of 95 percent is approached; at this point the consumption rises rapidly from this line and passes through the no-chemical pre-treatment point. Thus, at pretreatment yields above 95 percent, a disproportionately large amount of chlorine dioxide is consumed. This is, of course, very undesirable.

FIG. 7 shows a parallel behavior for bleached yields. At pretreatment yields above about percent, the bleached yield actually decreases to the point corresponding to mechanical pro-treatment only. Since maximum yield is desirable, this decrease is undesirable.

Further reason for maintaining pretreatment yield below 95 percent is found in Example [X, in which it is shown that percent pretreatment yield produces inferior paper strength properties as compared with papers in the below 95 percent pretreatment yield range.

EXAMPLE Vl To demonstrate the preferred conditions for the sodium base neutral sulfite pretreatment, it is shown below that proper pretreatment liquor impregnation of the chips prior to heating is necessary to maximize handsheet strength and minimize fiber bundle or shive content. It is also shown that proper ratio of pretreatment chemicals is necessary to provide maximum strength and ease of pulping and bleaching with the subsequent chlorine dioxide-caustic sequence.

Runs were made using two extremes of liquor impregnation prior to chemical pretreatment, and with a range of ratios of sodium sulfite to sodium carbonate in the pretreatment liquor. In all cases sufficient pretreatment chemical was applied so that the final liquor pH was above 7. If allowed to drop below this value, a weak pulp resulted. The raw material was a southern hardwood chip mixture. Following chemical pretreatment, the materials produced were refined under constant conditions in a laboratory eight-inch disc refiner, thoroughly water washed, and subjected to the fivestage chlorine dioxide-caustic sequence for pulping and bleaching to 80 GE. brightness (TAPPl Standard T 217 m-48). The total chlorine dioxide required to achieve this brightness was determined together with standard handsheet physical tests on the bleached pulp.

The critical experimental conditions, together with the indicated results, are shown below. Runs 1 and 2, at equal pretreatment yield, show the effect of variations in applied chemicals.

Pre treat- Pretreatment Liquor ment Impreg. 1: Sodium 11 Sodium Sodium Sulk Chlorine Std. b Handsheet Properties Time Before Max. Pre- Sulfite Carbonate file/Sodium Pre- Dioxide Re- Chlorine at 600CSF/300CSF Max. Temp., treatment Wood Wood Carbonate treatment quired Dioxide Tensile Burst Tear Wood Run (min) Temp., "F Basis Basis Ratio Yield Basis Required '1 I 1 I20 335 24 5 4.8 74 6.0 6.2 56/76 89/133 175/]36 2 335 l2 I!) 1.2 73 6.3 6.0 55/82 93/l65 208/l52 3 I20 335 l2 l0 L2 86 8.0 8.5 55/90 96/l78 208/l76 4 I20 335 9 9 L0 84 l0.l 8.! 49/71 78/!32 lS6/l25 -continued Pretreat- Pretreatment Liquor ment lmpreg. Sodium Sodium Sodium Sul- Chlorine Std. Handsheet Properties Time Before Max. Pre- Sulfite Carbonate fite/Sodium Pre- Dioxide Re- Chlorine at t500CSF/300CSF Max. Temp., treatment Wood Wood Carbonate treatment quired Dioxide Tensile Burst Tear Wood Run (min.) Temp., "F Basis Basis Ratio Yield Basis Required in it It 30 345 l4.2 5.2 2.7 85 8.3 8.3 35/74 62/135 2l0/l75 Values taken from the chlorine dioxide consumption pretreatment yield curve, Example IX. Fig. 8

'TAPPl Standard T404 "-66 'TAPPl Standard T403 63 TAPPl Standard T4l4 "-65 r TAPPl J T227 rn-58 While pulping chemical (chlorine dioxide) consumption is constant, a significant increase in pulp handsheet strengths results when the sulfite/carbonate ratio is reduced from about 5 to slightly more than 1.

Runs 3 and 4 show the effect of still further reduction in this base ratio from 1.2 to 1.0. A very significant decrease in pulp handsheet strengths results, along with a large increase (about 25 percent based on the normal chemical application) in chlorine dioxide consumption. Thus, while optimum conditions exist at a sulfate/carbonate ratio of slightly above unity, it is critical that this ratio not be allowed to drop below about 1.2 because of detrimental effects on both pulp physical properties and pulping chemical consumption.

Runs 1, 3 and 5 show the importance of adequate pretreatment liquor impregnation of the chipped raw material. Since the chemical ratio of Run 5 is intermediate between those of Runs 1 and 3, the observed marked decrease in handsheet physical properties is attributable to the lack of adequate chip impregnation. Optimum impregnation conditions depend upon the type (wood species) and chip dimensions of the raw material. Sufficient pretreatment chemical must be added to maintain the pH at 7 or above during the pretreatment.

The ratio of sodium sulfite to sodium carbonate applied should be held in the vicinity of about 1.5 for highest pulp strength. Higher values result in reduced strengths; values below about 1.2 give weaker pulps and increased pulping chemical (chlorine dioxide) consumption.

Adequate pretreatment liquor penetration into the raw material must be achieved, or strength properties nifying agents for at least a portion of this chlorine dioxide was investigated.

In general, with equivalent additions of bleaching chemicals, a higher bleached brightness is obtained using mixtures than with either chlorine or chlorine dioxide alone when bleaching kraft pulps. However, since the starting material with a bleachable kraft pulp is much different from the neutral sulfite-pretreated material presently under consideration, and since the desired action with chlorine and chlorine dioxide additions to conventional kraft pulp is largely one of bleaching (initial pulp lignin contents of perhaps 2 percent), while the chlorine dioxide in the present case is used to remove a large quantity of chemically different lignin (pretreated pulp lignin content approximately fifteen percent) as well as to bleach, prior art results are not directly translatable to the present invention.

Since some systems for chlorine dioxide generation result in the simultaneous generation of chlorine, and since efficient use of this chlorine is essential to sound chlorine dioxide generation economics, runs were made to determine the feasibility of substituting chlorine for chlorine dioxide at various points in the pulping sequence. Sodium base neutral sulfite pretreated southern hardwood chips were mechanically refined in a laboratory disc refiner and thoroughly water washed prior to the pulping-bleaching sequence. in one run all chlorine was substituted for the normally used chlorine dioxide. The chemical application and results in terms of pulp pentosan content, brightness, screen rejects (amount retained on a laboratory vibratory flat screen using an 0.008-inch cut screen) and handsheet brightness are summarized below:

'IAPPI Standard T230 :u-66; OP is degree of polymerization suffer and shive content increases. Optimum conditions depend upon raw material structure and particle dimensions.

EXAMPLE VII To demonstrate the extent to which chlorine can be substituted for chlorine dioxide in the present pulpingbleaching sequence, and because of potential economics, the desirability of substituting other selective delig- It is apparent from the extremely low brightness, low pentosan content, low DP, and high screen rejects that chlorine cannot be substituted for all chlorine dioxide.

Next a series of runs was conducted in which all and half of the chlorine dioxide normally applied in the first stage (equivalent oxidizing basis) was substituted by chlorine. The raw material again was a neutral sulfite pretreated southern hardwood chip mixture, mechanically refined in an eight-inch laboratory disc refiner; the pretreatment yield was percent. The conditions 19 employed, together with the brightness and strength results of handsheets prepared from the final pulps, are shown below:

EXAMPLE Vlll To demonstrate the advantages of using sodium base neutral sulfite (sodium sulfite and sodium carbonate) Firs pi ststagc chemical pretreatment in preference to either nitric in l i acid or ammonia base neutral sulfite, these three pre- Chlorim: Chlorine fi treatment combinations were evaluated. Dioxide Dioxide Chlorine The three chemical-mechanical pretreatments were Chlorine Dioxide in achieved using the chemical conditions shown below 'lajtgltage (wood basis) 4.7 2.35 0 using mixed southern hardwood chips as raw material.

orlne In Sl. mg: (wood basis) 0 6,15 123 In all cases, the chemical pretreatment was followed by is Sodium hydroxide in refining in a laboratory eight-inch disc refiner, thoreach of the lst and 2nd extraction stages 8 8 8 and pylplng a b.lea.ch1ng usfng qb chlorine dioxide in 2nd chlorine dloxlde-caustic-chlorme dloxlde-caustlc-chlochlcriflc dioxide a rine dioxide treatment sequence. Conditions and re- (wood basis) 2.4 2.4 2.4 It f H Chlorine dioxide in 3rd s were as 0 Sulfite, I: Carbonate, Equivalent Equivalent At 300II Canadian Pre- Sodium Sodium 71 Nitric Prelb Chlorine Standard Freeness' treatment Sulfite Carbonate Acid treatment Dioxide Bleached G.E. Tensile Burst Tear Chemical (on wood) (on wood) (on wood) Yield, (wood basis) Yield, Brightness I: '12

Sodium Base Neutral sulfite l2 l0 73 6.0 58 s0 82 I52 I65 Ammonia Base Neutral sulfite 13.2 ll.l 72 6.0 58 so 73 I43 I48 Nitric Acid 25 73 6.] 54 so 70 l2l H2 The neutral sulfite pretreatment cycle for both sodium and ammonia base involved 30 min. to and 60 min. at 273'F plus 30 min. to and I min. at 335'F.

'TAPPI Standard T217 m-48 TAI'PI Standard T404 ts-66 TAP?! Standard T403 ts-63 TAPPl Standard T4 ts-6S 'TAPPl Standard T227 m-SS chlorine dioxide stage 'TAPPI Standard T217 m4! 'TAPPI Standard T404 W66 'TAPPI Standard T403 ts-63 "TAPPI Standard T227 Ill-$3 From the above results it was concluded that there is no synergistic effect of chlorine-chlorine dioxide mixtures, as has been reported in the art for such mixtures when bleaching kraft pulps; brightness is somewhat lowered by 50 percent chlorine substitution (oxidizing equivalent basis), and is very significantly harmed by use of all chlorine in the first stage of the pulping sequence; substitution of all chlorine in the first stage results in unbleached, undefibered shives or fiber bundles; and strength properties are not greatly affected by the half-substitution case, but are very significantly reduced when all chlorine is used in the first stage.

While the use of chlorine in any proportion in the present invention offers no advantages in terms of pulp quality, certain amounts of chlorine can be used in the initial chlorine dioxide stage without adverse effects. The upper limit to chlorine substitution is about to percent of the total chlorine dioxide requirement on an equivalent oxidant basis; this maximum corresponds to a weight proportion of about 50 percent chlorine and 50 percent chlorine dioxide.

[t is apparent that nitric acid pretreatment results in both lower bleached yield and lower handsheet strength, indicating excessive carbohydrate degradation during this pretreatment. Sodium base neutral sulfite produces a stronger pulp than ammonia base pretreatment. Also, with the ammonia base pretreatment it is more difficult to delignify and bleach, and the pulp contains somewhat more shives and fiber bundles.

From this comparison, it is apparent that either of the neutral sulfite bases gives a better pretreatment than does nitric acid. Also, sodium base is superior to ammonia base neutral sulfite, other factors not considered. The ammonia base does give a very acceptable, high yield bleached pulp, and where factors such as pollution considerations are controlling, the ammonia base is exceptionally attractive.

EXAMPLE lX To demonstrate the advantages of using a neutral sulfite pretreatment in preference to a high-yield kraft pretreatment or an entirely mechanical pretreatment, the following runs were made.

The raw material was a mixture of southern hardwood chips (approximately one-third oak, one-third yellow poplar, and one-third gum). In the case of the chemical pretreatments (kraft and neutral sulfite), sufficient time for liquor impregnation was allowed prior to heating to maximum temperature. The chemically pretreated material (and the hardwood chips following presteaming in the case of mechanical pretreatment only) was then refined in an eight-inch laboratory disc refiner to give a starting material for the chlorine dioxide pulping/bleaching sequence. Pretreatment was analyzed through the following: 

1. IN THE PROCESS OF PRODUCING FROM WOOD A PAPER-MAKING PULP HAVING A YIELD OF FROM ABOUT 55 TO ABOUT 85% USING A PRELIMINARY PARTIAL DIGESTION FOLLOWED BY REFINING AND THEN DELIGNIFICATION WITH CIO2, THE IMPROVEMENT COMPRISING REFINING THE PARTIALLY DIGESTED WOOD IN A HIGH PRESSURE REFINED AT A TEMPERATURE ABOVE 212*F, AND SUBJECTING THE REFINED MATERIAL TO A DELIGNIFYING-BLEACHING SEQUENCE OF THREE OXIDIZING TREATMENTS WITH PREFORMED CIO2 ALTERNATING WITH TWO CASTIC EXTRACTION TREATMENTS TO BRING THE BRIGHTNESS OF THE PILP UP TO AT LEAST 80% G.E., THE AMOUNT OF CHLORINE DIOXIDE IN EACH OF THE CHLORINE DIOXIDE STAGES BEING PROGRESSIVELY SMALLER, AND THE TOTAL AMOUNT OF CHLORINE DIOXIDE USED BEING NOT MORE THAN ABOUT 9% BASED ON THE DRY WEIGHT OF THE ORIGINAL WOOD.
 2. The combination of claim 1 in which the partial digestion is a neutral sulfite digestion.
 3. In the process of producing high-quality paper-making pulp from wood by a preliminary partial digestion followed by refining and then delignification with ClO2, the improvement comprising partially digesting the wood to a pulp yield of between about 64 and about 85%, preceding the refining by dewatering, refining in a high pressure refiner at a temperature above 212*F, and subjecting the refined material to a delignifying-bleaching sequence of three oxidizing treatments with preformed ClO2 alternating with two caustic extraction treatments to bring the brightness of the pulp up to at least about 80% G.E., the amount of chlorine dioxide in each of the chlorine Dioxide stages being progressively smaller, and the total amount of chlorine dioxide used being not more than about 9% based on the dry weight of the original wood.
 4. The combination of claim 3 in which the wood is in the form of wood chips.
 5. The combination of claim 3 in which the refining is effected at a temperature of up to 340*F.
 6. The combination of claim 3 in which the partial digestion is a neutral sulfite digestion.
 7. The combination of claim 3 in which the digested product is dewatered and the refining is effected with a power input of from about 0.5 to about 5 horsepower-days per ton of pulp. 